To calculate a more specific estimate on the number of exoplanets in the galaxy, researchers start with the number of stars.

Problem is, no one’s certain about that either. But at least 100 billion stars are in the Milky Way, scientists say—maybe up to 400 billion.

Says Guillem Anglada-Escude, an astrophysicist at Queen Mary University of London, and part of the team that discovered Proxima b, Earth’s closest exoplanet: “100 billion is a reasonable number.”

So take 100 billion stars, assume three planets per star, and multiply: that's 300 billion planets in the galaxy.

Presume more stars—200 billion? 300 billion?—and “you’re looking at close to a trillion planets,” says David Kipping, astronomy professor at Columbia University and the head of its Cool Worlds laboratory.

“That’s as good as any guess right now. It could be a trillion, it could be more than that.”

The rocky exoplanet HD 219134b.Credit: NASA / JPL-Caltech

A bit trickier: figuring out how many of those planets might orbit within the habitable zone.

That’s the place in a star system where a world's surface temperature is moderate, where liquid water might flow, where life has a chance.

Scientists generally agree a significant fraction of exoplanets reside in the zone, perhaps anywhere from five to fifteen percent.

“That's tens of billions of planets in the galaxy with some potential for life,” says Margot.

Astronomers, exceedingly precise on almost everything else, aren’t particularly bothered by the broad range of approximations. When it comes to exoplanets, what counts is this: all the numbers are big. Really big.

“As humans, we can’t even conceive what billions and billions are,” says Sara Seager, astrophysicist and planetary scientist at MIT.

“But they’re all huge numbers, right? We can agree on that.”

And that’s just the neighborhood, says Margot. Beyond the Milky Way loom “two trillion galaxies in the observable universe," each with incalculable billions of stars.

And that's how many planets?

For this one, Margot doesn't suggest a number.

“It's comparable,” he says, “to the number of grains of dry sand on all the beaches on Earth.” Imagine counting that.

I’m a multimedia journalist who loves to write about science—especially space. As a writer and producer of TV and radio documentaries, I’m a four-time Emmy Award winner, the recipient of the Edward R. Murrow Award for Excellence in Writing, a five-time winner of first place...